This application is based on Japanese Patent Application No. 2001-007274 filed on Jan. 16, 2001, the contents of which are incorporated hereinto by reference.
1. Field of the Invention
The present invention relates in general to an electric-component mounting system, and more particularly to techniques for improving accuracy of mounting of electronic components and other electric components.
2. Discussion of Related Art
An electric-component mounting system is constructed and arranged to mount various kinds of electric components on a board or substrate such as printed-wiring boards. The electric components include resistors, electronic components of chip type such as capacitors, electronic components of flat-package type, and various other types of electronic components with or without leads, such as connectors
An electric-component mounting system of such type is generally arranged to hold each electric component by suction, move and position the electric component to and at a predetermined position above a substrate on which the electric components are to be mounted, and mount or place the electric component on the substrate.
Described in detail, the electric-component mounting system of the type described above includes (a) a main body, (b) a movable portion, (c) a drive device, (d) a motion-transmitting member which extends along a straight line and which is operated by the drive device to apply a linear motion along the straight line, to the movable portion, and (e) a controller which is arranged to apply a drive signal to the drive device for controlling the drive device, so as to control the position of the movable portion in the direction of its linear motion. For example, the motion-transmitting member is a ballscrew which is rotated by the drive device such that the ballscrew is not axially movable and such that a rotary motion of the ballscrew is converted into a linear motion of the movable portion. Another example of the motion-transmitting member is a non-rotatable ballscrew which meshes with a ballnut such that a rotary motion of the ballnut is converted into a linear motion of the movable portion. A further example of the motion-transmitting member is a linear stator which slidably engages a slider and whose magnetic force is converted into a linear motion of the slider. The stator and the slider cooperate to constitute a linear motion.
For instance, the electric-component mounting system described above may be arranged such that the movable portion includes a component holder arranged to hold an electric component by suction, and the main body has an image-taking device fixed thereto, to take an image of the electric component as held by the component holder. The controller indicated above is arranged to process the image of the electric component taken by the image-taking device, for detecting an actual hold-position of the electric component as held by the component holder. The controller obtains a positioning error of the electric component, which is an error of the detected actual position of the electric component with respect to a nominal hold-position of the electric component. The controller is further arranged to determine the drive signal to be applied to the drive device, so that the actual mounting position of the electric component is not influenced by the obtained positioning error of the electric component.
The electric-component mounting system described above may be arranged such that the movable portion includes a movable member which is moved relative to the substrate and which carries an another image-taking device fixed thereto, so that this image-taking device is moved with the movable member, to take an image of a fiducial mark provided on the substrate. The controller processes the image of the fiducial mark taken by this image-taking device, to detect an actual position of the substrate, and obtains a positioning error of the substrate, which is an error of the detected actual position with respect to a nominal position of the substrate.
Alternatively, the electric-component mounting system may be arranged to include a plurality of movable portions in the form of a component holder for holding an electric component, and a movable member, and further include a first image-taking device fixed on the main body, and a second image-taking device fixed on the movable member. The first image-taking device is arranged to an image of the electric component as held by the component holder, while the second image-taking device is arranged to taken an image of a fiducial mark provided on the substrate. In this case, the controller indicated above is arranged to process the image of the component taken by the first image-taking device, for detecting the actual hold-position of the electric component as held by the component holder, and obtaining the positioning error of the electric component, which is an error of the detected actual position of the electric component with respect to the nominal hold-position of the electric component. The controller is further arranged to process the image of the fiducial mark taken by the second image-taking device, for detecting the actual position of the substrate, and obtaining the positioning error of the substrate, which is an error of the detected actual position with respect to the nominal position of the substrate. The controller is further arranged to determine the drive signal to be applied to the drive device, so that the actual mounting position of the electric component is not influenced by the obtained two positioning errors, that is, the positioning errors of the electric component and the substrate.
In the electric-component mounting systems which have been described, the drive device usually includes servo-amplifiers, electric motors and other electric devices that generate operating heat, and the movable portion and the motion-transmitting member generate friction heat due to relative movement therebetween. Accordingly, the main body and the motion-transmitting member are inevitably heated and subject to thermal expansion. This thermal expansion causes deterioration of accuracy of mounting of the electric component by the electric-component mounting system.
For reducing the deterioration of the mounting accuracy of the electric component due to the thermal expansion, it has been a conventional practice to perform a warm-up or idling operation of the electric-component mounting system prior to each production run of the system, for positively inducing the thermal expansion of the system. This warm-up operation is desirably continued until the amount of thermal expansion of the system is substantially saturated.
Conventionally, an image of the fiducial mark is taken by the appropriate image-taking device described above, after the warm-up operation, to detect the actual position of the fiducial mark, that is, to obtain the positioning error of the substrate, namely, an error of the actual position of the substrate with respect to the nominal position. In the subsequent production run of the system, the drive signal to be applied to the drive device is compensated for the positioning error obtained on the basis of the image of the fiducial mark immediately after the warm-up operation.
The conventional compensation of the drive signal described above is effective to reduce an influence of the positioning error due to the thermal expansion of the electric-component mounting system, on the actual mounting position of the electric component on the substrate. However, the conventional compensation is not satisfactory to perfectly eliminate the influence of the positioning error due to the thermal expansion. This aspect will be described in detail with respect to the thermal expansion of the ballscrew used as the motion-transmitting member.
Where the positioning error of the fiducial mark is detected on the basis of the image of the mark taken by the image-taking device, as described above, the detected positioning error include an error component due to the thermal expansion of the ballscrew, and the other error component, that is, the positioning error of the substrate per se. These error components cannot be distinguished from each other.
Therefore, the conventional compensation does not a permit accurate positioning of the electric component at the nominal mounting position on the substrate, by correct compensation of the mounting position for the actual mount of the thermal expansion.
It is noted that the positioning error due to the thermal expansion is not constant at different positions along the axis of the ballscrew. Where the ballscrew has a considerably small length, the mounting accuracy of the electric component would not be considerably influenced by a variation of the positioning error, even if the positioning error were assumed to be constant over the entire length of the ballscrew. Where the ballscrew has a relatively large length, however, it is not adequate, for assuring high mounting accuracy of the electric component, to ignore the variation of the positioning error due to the thermal expansion of the ballscrew, which variation takes place in the axial direction of the ballscrew.
The conventional compensation is effected without taking account of the dependency of the thermal expansion amount on the mounting position of the electric component on the substrate, in the axial direction of the ballscrew. Accordingly, the conventional compensation suffers from deterioration of the mounting accuracy of the electric component due to the thermal expansion, particularly where the ballscrew has a relatively large length.
Further, the conventional compensation requires the electric-component mounting system to perform the warm-up or idling operation prior to the production run, undesirably reducing the productive time of the system and lowering the operating efficiency of the system.
It is therefore an object of the present invention to provide an electric-component mounting system capable of mounting an electric component on the substrate, with a high degree of positioning accuracy of the electric component, irrespective of thermal expansion of the system. This object may be achieved according to any one of the following modes of the present invention, each of which is numbered like the appended claims and depends from the other mode or modes, where appropriate, to indicate and clarify possible combinations of elements or technical features. It is to be understood that the present invention is not limited to the technical features or any combinations thereof which will be described for illustrative purpose only. It is to be further understood that a plurality of elements or features included in any one of the following modes of the invention are not necessarily provided all together, and that the invention may be embodied without some of the elements or features described with respect to the same mode.
(1) An electric-component mounting system wherein an electric component held by a component holder is moved to and positioned at a target mounting position on a substrate, and the positioned electric component is mounted on the substrate, the electric-component mounting system comprising:
a main body structure;
a drive device;
a movable portion movable relative to the main body structure;
a motion-transmitting member disposed on the main body and linearly extending in one direction, the motion-transmitting member being operable to transmit to the movable portion a linear motion in the one direction generated by an operation of the drive device, such that thermal expansion of the motion-transmitting member causes a corresponding positioning error of the movable portion in the direction of the linear motion;
an object fixedly disposed on one of the main body structure and the movable portion;
an image-taking device fixedly disposed on the other of the main body structure and the movable portion and operable to take an image of the object, the object and the image-taking device being positioned relative to each other such that an error of relative positioning of the object and the image-taking device, which is detected on the basis of the image of the object taken by the image-taking device substantially represents a thermal-expansion positioning error which is a positioning error of the object which is derived from thermal expansion of the electric-component mounting system; and
a controller operable to apply a drive signal to the drive device, for controlling a position of the movable portion in the direction of the linear motion, the controller determining the drive signal on the basis of the image of the object taken by the image-taking device, so as to reduce an amount of influence of the thermal-expansion positioning error on an actual position of the movable portion in the direction of the linear motion.
In the electric-component mounting system according to the above mode (1) of this invention, the object to be imaged is disposed on one of the main body structure of the system and the movable portion, while the image-taking device to take the image of the object is disposed on the other of the body portion and the movable portion. These object and image-taking device are positioned relative to each other such that the error of positioning of the object relative to the image-taking device, which is detected on the basis of the image of the object taken, substantially represents the thermal-expansion positioning error of the object derived from the thermal expansion of the electric-component mounting system.
Further, the controller determines the drive signal to be applied to the drive device, on the basis of the image of the object taken by the image-taking device, so as to reduce the amount of influence of the thermal-expansion positioning error of the object on the actual position of the movable portion in the direction of its linear motion.
In the electric-component mounting system constructed as described above, the image-taking device alone can detect the thermal-expansion positioning error of the object, so that the electric component can be mounted on the substrate or board with a sufficiently high degree of positioning accuracy, irrespective of the thermal expansion of the electric-component mounting system.
As is apparent from the foregoing explanation, the accuracy of positioning of the electric component as mounted on the substrate is influenced by not only the thermal expansion of the motion-transmitting member but also the thermal expansion of the main body structure of the electric-component mounting system. In the electric-component mounting system according to the above mode (1) of the present invention, the controller is operable to determine the drive signal for the drive device, by taking into account not only a positioning error of the movable portion due to the thermal expansion of the motion-transmitting member, but also a positioning error of the movable portion due to the thermal expansion of the main body structure. The present arrangement permits a sufficiently high degree of positioning accuracy of the electric component as mounted on the substrate, irrespective of the thermal expansion of the motion-transmitting member and main body structure. However, the controller may be arranged to determine the drive signal, by taking account of only one of the two positioning errors of the movable portion which are derived from the thermal expansion of the motion-transmitting member and the thermal expansion of the main body structure, respectively.
For the controller to be able to optimize the drive signal, one of the object and the image-taking device which is fixedly disposed on the main body structure is desirably located at a portion of the main body structure at which the amount of thermal expansion is smaller than at the other portion (e.g., a portion at which the motion-transmitting member is supported), so that the position and attitude of the object or image-taking device disposed on the main body structure is less likely to be influenced by the thermal expansion of the main body structure.
In the present electric-component mounting system, the motion-transmitting member may be arranged to move one movable portion, or a plurality of movable portion.
The term xe2x80x9cthermal expansion of the electric-component systemxe2x80x9d used herein is interpreted to comprehend at least one of the thermal expansion of the main body structure and the thermal expansion of the motion-transmitting member.
(2) An electric-component mounting system according to the above mode (1), wherein the image-taking device is fixedly disposed on the main body structure, at a position at which the image-taking device is not substantially influenced by the thermal expansion of the electric-component mounting system, while the object is fixedly disposed on the movable portion, at a position at which the object is influenced by the thermal expansion of the electric-component mounting system.
In the electric-component mounting system according to the above mode (2), the object and the image-taking device are positioned relative to each other as required according to the above mode (1), that is, such that the error of positioning of the object relative to the image-taking device, which is detected on the basis of the image of the object taken by the image-taking device, substantially represents the thermal-expansion positioning error which is a positioning error of the object which is derived from the thermal expansion of the electric-component mounting system.
The image-taking device may be disposed on the main body structure, at a position at which the image-taking device is not substantially influenced by not only the thermal expansion of the electric-component mounting system but also other factors of the system. In this case, the object is disposed on the movable portion, at a position at which the object is influenced by the thermal expansion of the system but is not substantially influenced by the other factors. For instance, the xe2x80x9cother factors of the systemxe2x80x9d relating to the object include a positioning error of the electric component as held by the component holder which is carried by the movable portion, as described below with respect to the following mode (3).
(3) An electric-component mounting system according to the above mode (1) or (2), wherein the movable portion carries the component holder operable to hold the electric component by suction, and the object is fixedly disposed on the movable portion, while the image-taking device is fixedly disposed on the main body structure, and is operable to take not only the image of the object but also an image of the electric component as held by the component holder.
In the electric-component mounting system according to the above mode (3) wherein the image-taking device is arranged to take not only the image of the object but also the image of the electric component, the required number of image-taking devices can be made smaller than in a system which uses two image-taking devices for taking the images of the object and electric component, respectively.
(4) An electric-component mounting system according to any one of the above modes (1)-(3), wherein the movable portion includes a first movable portion, and a second movable portion which carries the component holder operable to hold the electric component by suction, and the motion-transmitting member includes a first motion-transmitting member and a second motion-transmitting member which are operable to move the first and second movable portions, respectively, and which extend in respective directions intersecting each other, the first motion-transmitting member being directly mounted at one of opposite ends thereof on the main body structure, while the second motion-transmitting member being mounted at one of opposite ends thereon on the first movable portion and indirectly mounted on the main body structure, the object being fixedly disposed on the second movable portion.
In the system according to the above mode (4), the first movable portion is movable in a first direction parallel to the direction of extension of the first motion-transmitting member, while the second movable portion is movable in the first direction and a second direction which is parallel to the direction of extension of the second motion-transmitting member and which intersects the first direction. Further, the component holder and the object to be imaged by the image-taking device are both provided on the second movable portion, so that the component holder and the object are always moved together.
In the system described above, therefore, both of the thermal expansion of the first motion-transmitting member in its direction of extension and the thermal expansion of the second motion-transmitting member in its direction of extension can be detected on the basis of the image of the object taken by the image-taking device, so that the component holder can be moved so as to minimize an influence of those thermal expansions, by determining the drive signal on the basis of the detected thermal expansions.
In one form of the system according to the above mode (4), the main body structure has a portion which extends in a direction (e.g., X-axis direction described below) intersecting the direction of extension (e.g., Y-axis direction) of the first motion-transmitting member, and which portion comparatively easily suffers from thermal expansion in its direction of extension. Further, the first motion-transmitting member is directly mounted at its one end on that portion of the main body structure, and the second motion-transmitting member extends in parallel with the direction of extension of the above-indicated portion of the main body structure and has a smaller length than the first motion-transmitting member so that the second motion-transmitting member is less likely to suffer from the thermal expansion than the first motion-transmitting member. In this form of the system, the mounting accuracy of the electric component is influenced by a positioning error due to its thermal expansion in its direction of extension (Y-axis direction), and a positioning error due to the thermal expansion of the above-indicated portion of the main body structure in its direction of extension (X-axis direction). Accordingly, the controller is arranged to eliminate or reduce the deterioration of the mounting accuracy of the electric component due to one or both of the positioning errors caused by the thermal expansions in the above-indicated two directions (Y-axis and X-axis directions). This form of the system is applicable to the system according to the following mode (5).
(5) An electric-component mounting system according to any one of the above modes (1)-(3), wherein the movable portion includes a first movable portion, and a second movable portion which carries the component holder operable to hold the electric component by suction, and the motion-transmitting member includes a first motion-transmitting member and a second motion-transmitting member which are operable to move the first and second movable portions, respectively, and which extend in respective directions intersecting each other, the first motion-transmitting member being directly mounted at one of opposite ends thereof on the main body structure, while the second motion-transmitting member being mounted at one of opposite ends thereon on the first movable portion and indirectly mounted on the main body structure, the object being fixedly disposed on the second movable portion.
In the system according to the above mode (5), the first movable portion is movable in a first direction parallel to the direction of extension of the first motion-transmitting member, while the second movable portion is movable in the first direction and a second direction which is parallel to the direction of extension of the second motion-transmitting member and which intersects the first direction, as in the system according to the above mode (4). Unlike the system according to the above mode (4), however, the system according to the above mode (5) is arranged such that the object is not provided on the second movable portion which carries the component holder, but is provided on the first movable portion. Accordingly, the object is movable only in the direction of extension of the first motion-transmitting member, in the system according to the above mode (5).
In the system according to the above mode (5), therefore, the thermal expansion of the first motion-transmitting member in the first direction can be detected on the basis of the image of the object provided on the first movable portion, but the thermal expansion of the second motion-transmitting member in the second direction cannot be detected on the basis of the image of the object.
Where the two motion-transmitting members extending in the mutually intersecting directions are used to move the component holder, it is not necessarily equally important to detect the thermal expansions of the two motion-transmitting members. Generally, the first motion-transmitting member directly mounted on the main body structure has a larger length and accordingly suffers from a larger amount of thermal expansion than the second motion-transmitting member. For this reason, in particular, it is more important to detect the thermal expansion of the first motion-transmitting member than to detect the thermal expansion of the second motion-transmitting member. In some cases, the mounting accuracy of the electric component is not significantly deteriorated even if the thermal expansion of the second motion-transmitting member is not taken into account when the drive signal for the drive device is determined.
If the object is provided on the second movable portion where the detection of the thermal expansion of the second motion-transmitting member is not so important, the provision of the object on the second movable member merely results in an increase in the total weight of the second movable portion and members provided to hold the object on the second movable portion, undesirably increasing a tendency of lowering a kinetic response of the component holder to not only the drive signal to move the component holder in the second direction, but also the drive signal to move the component holder in the first direction. In this case, the second movable portion must be always moved together with the object and the members used to hold the object, as well as the component holder.
Where the object is provided on the first movable portion, on the other hand, the total weight of the first movable portion and members used to hold the object on the first movable portion is increased, and this increased total weight tends to deteriorate the kinetic response of the component holder to the drive signal to move the component holder in the first direction, but does not deteriorate the kinetic response to the drive signal to move the component holder in the second direction. Namely, the component holder alone is moved in the second direction.
Therefore, the system according to the above mode (5) wherein the object is provided on the first movable portion is advantageous in that it is possible to reduce the deterioration of the kinetic response of the component holder to the drive signals, where the detection of the thermal expansion of the first motion-transmitting member is more important than the detection of the thermal expansion of the second motion-transmitting member.
(6) An electric-component mounting system according to any one of the above modes (19-(3), wherein the movable portion includes a first movable portion, and a second movable portion which carries the component holder operable to hold the electric component by suction, and the motion-transmitting member includes a first motion-transmitting member and a second motion-transmitting member which are operable to move the first and second movable portions, respectively, and which extend in respective directions intersecting each other, the first motion-transmitting member being directly mounted at one of opposite ends thereof on the main body structure, while the second motion-transmitting member being mounted at one of opposite ends thereon on the first movable portion and indirectly mounted on the main body structure, the object consisting of two objects fixedly disposed on the first and second movable portions, respectively.
In the system according to the above mode (6), the thermal expansion which influences the second motion-transmitting member includes a first component derived from the thermal expansion of the second motion-transmitting member per se (which first component varies depending upon the position along the length of the second motion-transmitting member, for example), and a second component derived from the thermal expansion of a portion of the main body structure at which the first motion-transmitting member is fixedly supported at its proximal end (which second component does not vary depending upon the position along the length of the second motion-transmitting member).
In the system according to the above mode (6), the two objects are provided for the respective two motion-transmitting members, so that the first component of the thermal expansion can be detected on the basis of the image of the object provided for the second motion-transmitting member, while the second component can be detected on the basis of the image of the object provided for the first motion-transmitting member.
As described above, the provision of the two objects for the respective two motion-transmitting members permits detections of the two components of the thermal expansion influencing the second motion-transmitting member, independently of each other. The two components are different from each other in that the first component varies depending upon the position along the length of the second motion-transmitting member, while the second component does not vary depending upon this position. It is important to take account of this difference when the drive signal to move the second movable member is controlled by the controller, so as to prevent or reduce the influence of the thermal expansion on the mounting accuracy of the electric component.
However, the provision of the two objects for the respective two motion-transmitting members is not essential to detect the above-indicated two components of the thermal expansion which influences the second motion-transmitting member. Namely, the two components can be detected by providing only the second movable portion with the object, and providing a first image-taking device arranged to take an image of the object when the second movable portion is located at a position sufficiently near the proximal end of the second motion-transmitting member, that is, at a position at which position of the second movable is considerably influenced by the thermal expansion of the main body structure), and a second image-taking device arranged to take an image of the object when the second movable portion is located at a position which is sufficiently spaced from the proximal end of the second motion-transmitting member, that is, at a position at which the position of the second movable portion is considerably influenced by the thermal expansion of the second motion-transmitting member. In this form of the system, the second component can be detected on the basis of the image of the object taken by the first image-taking device, and the first component can be detected on the basis of the image of the object taken by the second image-taking device.
(7) An electric-component mounting system according to the above mode (1), wherein the object is fixedly disposed on the main body structure, at a position at which the object is not substantially influenced by the thermal expansion of the electric-component mounting system, while the image-taking device is fixedly disposed on the movable portion, at a position at which the image-taking device is influenced by the thermal expansion of the electric-component mounting system.
In the electric-component mounting system according to the above mode (7), too, the object and the image-taking device are positioned relative to each other as described above with respect to the above mode (1). The image-taking device may be disposed on the main body structure, at a position at which the image-taking device is not substantially influenced by not only the thermal expansion of the electric-component mounting system but also other factors of the system. In this case, the object is disposed on the movable portion, at a position at which the object is influenced by the thermal expansion of the system but is not substantially influenced by the other factors. For instance, the xe2x80x9cother factors of the systemxe2x80x9d relating to the object in this mode (7) include a positioning error of the substrate.
(8) An electric-component mounting system according to any one of the above modes (1)-(7), wherein the movable portion includes a movable member which is movable relative to the substrate and which carries the image-taking device, the image-taking device being moved with the movable member to take an image of a fiducial mark provided on the substrate, as well as the image of the object, the object being fixedly disposed on the main body structure.
In the electric-component mounting system according to the above mode (8), the image-taking device is operable to take not only the image of the object but also the image of the fiducial mark provided on the substrate. Accordingly, the required number of image-taking devices can be made smaller than in a system which uses two image-taking devices for taking the images of the object and the fiducial mark, respectively.
(9) An electric-component mounting system according to the above mode (1), wherein the movable portion consists of a plurality of movable portions at least one of which includes the component holder operable to hold the electric component by suction, at least one of the other of the plurality of movable portions including a movable member movable relative to the substrate, for the image-taking device to take an image of a fiducial mark provided on the substrate,
the object consisting of a plurality of objects including at least one first object fixedly disposed on the at least one movable portion, and at least one second object which corresponds to the at least one of the other of the plurality movable portions and which is fixedly disposed on the main body structure,
the image-taking device consisting a plurality of image-taking devices including at least one first image-taking device which corresponds to the at least one movable portion and each of which is fixedly disposed on the main body portion and operable to take not only an image of the at least one first object but also an image of the electric component held by the component holder, the plurality of image-taking device further including at least one second image-taking device which is fixedly disposed on the at least one of the other of the plurality of movable portions, for taking not only an image of the at least one second object but also the image of the fiducial mark,
and wherein the controller determines the drive signal, on the basis of the images of the at least one first object and the electric component taken by the at least one first image-taking device and the images of the at least one second object and the fiducial mark taken by the at least one second image-taking device, so as to reduce the amount of influence of the thermal-expansion positioning error on the actual position of each of the plurality of movable portions in the direction of the linear motion.
(10) An electric-component mounting system according to any one of the above modes (1)-(9), comprising a plurality of positioning devices each of which consists of the movable portion, the drive device and the motion-transmitting member, and wherein a set of the object and the image-taking device is provided for each of the plurality of positioning devices.
In the electric-component mounting system according to the above mode (10), the positioning error of each movable portion due to the thermal expansion can be detected. Accordingly, it is possible to deal with the thermal expansion at each of the motion-transmitting members and the thermal expansion at each of the portions of the main body structure at which the motion-transmitting members are mounted.
(11) An electric-component mounting system according to any one of the above modes (1)-(10), wherein one of the object and the image-taking device which is fixedly disposed on the main body structure is provided at a plurality of positions which are spaced apart from each other in the direction of extension of the motion-transmitting member.
The electric-component mounting system according to any one of the above-described modes (1)-(10) may be arranged such that only one object or image-taking device is fixedly disposed on the main body structure. In this form of the system, the controller determines the drive signal, on an assumption that the amount of thermal expansion at a reference point established along the length of the motion-transmitting member is zero. For instance, the reference point is located at the proximal end of a ballscrew provided as the motion-transmitting member, at which the ballscrew is fixedly supported. In this case, the controller may be arranged to estimate the amount of thermal expansion at a given axial position of the motion-transmitting member, on the basis of the image of the object taken by the image-taking device at that axial position spaced from the reference point in the axial direction of the motion-transmitting member, on the above-described assumption, and on an assumption that the amount thermal expansion is proportionally increased with a distance of the above-indicated axial position from the reference point.
However, the amount of thermal expansion at the reference point is not actually necessarily zero. Further, the amount of thermal expansion at an axial position of the motion-transmitting member may not be actually proportionally increased with the distance of that axial position from the reference point.
In the electric-component mounting system according to the above mode (11), the two or more objects or image-taking devices are fixedly disposed on the main body structure such that the objects or image-taking devices are arranged in the direction of extension or axial direction of the motion-transmitting member.
In the present system, therefore, the amounts of thermal expansion can be detected at different axial positions of the motion-transmitting member, so that the amount of thermal expansion at a given axial position of the motion-transmitting member can be detected or estimated with a higher degree of accuracy in the present system than in a system in which only one object or only one image-taking device is disposed on the main body structure.
(12) An electric-component mounting system according to any one of the above modes (1)-(11), wherein the object has a central portion and a peripheral portion which are imaged by the image-taking device such that the central portion and the peripheral portion can be distinguished from each other, the central portion and the peripheral portion lie in respective two parallel planes which are spaced from the image-taking device by respective different distances when the image of the object is taken by the image-taking device, the central portion lying on one of the two parallel planes which is nearer to the image-taking device than the other plane.
In the electric-component mounting system according to the above mode (12) wherein the central and peripheral portions of the object lie in respective two parallel planes, the central and peripheral portions can be imaged such that these portions are distinguished from each other with different degrees of contrast, which are established by mechanical means, that is, by relative positioning of the central and peripheral portions with respect to the image-taking device.
Accordingly, the controller can detect the position of the object with a higher degree of accuracy on the basis of the positions of the clearly distinguished central and peripheral portions, making it possible to improve the accuracy of imaging of the object and accordingly improve the accuracy of detection of the positioning error of the movable portion on the basis of the image of the object.
(13) An electric-component mounting system according to the above mode (12), wherein the central portion has a surface having a lower value of brightness than a surface of the peripheral portion.
(14) An electric-component mounting system according to the above mode (13), wherein the surface of the central portion has a lower value of light reflectance than the surface of the peripheral portion.
(15) An electric-component mounting system according to the above mode (13), wherein the surface of the central portion does not emit a light while the surface of the peripheral portion emits a light.
(16) An electric-component mounting system according to any one of the above modes (12)-(15), wherein the object includes a main body, and a projecting portion extending from a surface of the main body, the central portion consisting of a distal end face of the projecting portion, while the peripheral portion consisting of a portion of the surface of the main body which surrounds a proximal end of the projecting portion.
(17) An electric-component mounting system according to the above mode (16), wherein the end face of the projecting portion has a circular shape.
In the electric-component mounting system according to the above mode (17), the projecting portion has a circular end face which functions as the central portion of the object. The circular central portion need not be positioned in its circumferential direction, so as to establish a predetermined angular position. In other words, the angular position of the circular central portion does not have an influence on the image of the central portion taken by the image-taking device, and the controller need not take into account the angular position of the central portion of the object, in determining the drive signal for the drive device so as to assure high positioning accuracy of the electric component as mounted on the substrate.
(18) An electric-component mounting system according to the above mode (16) or (17), wherein the end face of the projecting portion has an outer profile located outwardly of an outer profile of the proximal end, as seen in a direction in which the image of the object is taken by the image-taking device.
In the electric-component mounting systems according to the above modes (16) and (17), the projecting portion may be a cylindrical member having a constant diameter over its entire length. In this case, the outer profile of the distal end face of the projecting portion is aligned with that of the proximal end of the projecting portion, as seen in the direction in which the image of the object is taken by the image-taking device. In this arrangement, however, the boundary between the central and peripheral portions of the object may be obscure, if the outer profile of the distal end face is partly or totally located inwardly of the outer profile of the proximal end of the projecting portion, for some reason or other, for instance, due to a dimensional error of the projecting portion during its manufacture. In this case, the drive signal cannot be suitably determined by the controller, on the basis of the image of the object taken by the image-taking device.
In the electric-component mounting system according to the above mode (18), however, the outer profile of the distal end face of the projecting member is located outwardly of the outer profile of the proximal end.
This arrangement of the projecting portion is effective to prevent deterioration of the clarify of the boundary between the central and peripheral portions of the object due to the dimensional error of the projecting portion, or for any other reasons, in the image of the object taken by the image-taking device.
Where the peripheral portion is provided by a planar element which covers the above-indicated portion of the surface of the main body and the thickness of which is substantially zero, the proximal end of the projecting portion is substantially flush with the surface of the planar element. Where the thickness of the planar element providing the peripheral portion is not substantially zero, the projecting portion may be a projecting part of a projecting member which has a proximal end portion located within the thickness of the planar element. In this case, the proximal end of the projecting portion is the proximal end of the above-indicated projecting part of the projecting member.
(19) An electric-component mounting system according to the above mode (18), wherein the peripheral portion is provided by an adhesive-backed layer attached to the above-indicated portion of the surface of the main body, and the projecting portion is a projecting part of a projecting member, the projecting part having the distal end face of the projecting portion, and a proximal end face opposite to the distal end face, the projecting member including a proximal end part having a smaller size in transverse cross section than the projecting part, the projecting member having a shoulder surface formed between the proximal end part and the proximal end face of the projecting part, the adhesive-backed layer having a through-hole in which the proximal end part is fitted such that the shoulder surface is held in contact with a portion of the adhesive-backed layer in which the through-hole is formed.
In the electric-component mounting system according to the above mode (19), the peripheral portion of the object is provided by an adhesive-backed layer, and the projecting portion is a projecting part of a projecting member, which projecting part has the distal end face of the projecting portion and a proximal end face opposite to the distal end face. The adhesive-backed layer has a through-hole through which the proximal end part of the projecting member extends for attachment to the main body. Since the size of the proximal end part fitted in the through-hole is smaller than that of the projecting part having the distal end face providing the central portion, a dimensional error of the through-hole during its manufacture does not obscure the boundary between the central and peripheral portions of the object. In other words, the periphery of the distal end face of the projecting part in the image of the object taken by the image-taking device is not obscured by the periphery of the through-hole even if the actual size of the through-hole is larger than the nominal size due to a dimensional error of the through-hole.
(20) An electric-component mounting system according to any one of the above modes (1)-(19), wherein the controller includes imaging-frequency control means for operating the image-taking device to take the image of the object more frequently when a rate of change of the thermal-expansion positioning error is relatively high than when the rate of change is relatively low.
In the electric-component mounting system according to the above mode (20), the imaging-frequency control means prevents unnecessarily frequent operation of the image-taking device to take the image of the object for detecting the thermal expansion, making it possible to reduce the deterioration of the mounting accuracy of the electric component due to the thermal expansion, without sacrificing the operating efficiency of the electric-component mounting system.
(21) An electric-component mounting system according to any one of the above modes (1)-(20), wherein the controller includes proportional-type drive-signal determining means for determining the drive signal, so as to reduce the amount of influence of the thermal-expansion positioning error on the actual position of the movable portion in the direction of the linear motion, on an assumption that an amount of thermal expansion of the motion-transmitting member at a given position in the direction of the linear motion is proportionally increased with a distance of the given position from a predetermined reference point established on the motion-transmitting member in the direction of the linear motion.
In the electric-component mounting system according to the above mode (21), the drive signal to be applied to the drive device can be suitably determined by the controller, where the motion-transmitting member is fixedly supported at one of its opposite ends, since the controller determines the drive signal, in view of a tendency that the amount of thermal expansion of the motion-transmitting member at a given axial position thereof is proportionally increased as the distance of the axial position from the predetermined reference point is increased.